Low level alkali metal detection in combustion gas streams

Abstract

Alkali metal detectors previously employed in coal gasification systems are not suitable for use in combustion gas streams because of the high temperatures involved. In the present invention, alkali metal detector elements are mounted on a heat resistant flange including at least one shielded electrical ceramic feed-through. This flange is cooled to ensure the nonconductivity of the ceramic insulation.

The invention claimed is: 
     
       1. A device for detecting the presence of one or more alkali metals present in trace amounts in a flowing hot gas, comprising: a metaliferrous filament having a work function of at least 5.3 electron volts;   an electrically conductive ion collector disposed at a substantially constant distance from said filament so as to define an ionization collection region therebetween, said collector having a plurality of apertures therein for the passage of said gases;   means for applying a high voltage, direct current potential difference between said collector and said filament, whereby alkali metal ions formed at said filament are urged toward said collector where neutralization of said ions occurs, resulting in current through said collector;   a heat resistant support flange having insulated electrical feed-through support rods disposed therethrough for supporting said collector and said filament, said feed-through insulation, at least for said collector support rod, comprising ceramic material; and   means for cooling said ceramic material.   
     
     
       2. The device of claim 1 in which said rod supporting said collector is disposed through a shielded feed-through in said flange. 
     
     
       3. The device of claim 2 in which said shield feed-through comprises a pair of ceramic plugs between which is coaxially disposed an electrically conductive sleeve shield. 
     
     
       4. The device of claim 1 in which said filament is selected from the group consisting of platinum, tungsten, rhenium, paladium, and the oxides and alloys thereof. 
     
     
       5. The device of claim 1 in which said means for cooling comprises at least one fluid conduit within said flange together with means for circulating cooling fluid therethrough. 
     
     
       6. The apparatus of claim 1 further including an insulated feed-through in said flange and a thermocouple sensor disposed therethrough. 
     
     
       7. The device of claim 1 further including means for removing alkali metal contaminants from said filament. 
     
     
       8. The device of claim 7 in which said alkali removing means comprises means to resistively heat said filament. 
     
     
       9. The device of claim 1 in which said ion collector comprises a cylindrically shaped screen at least partially surrounding said filament and disposed in a coaxial relationship therewith. 
     
     
       10. The device of claim 1 in which said filament comprises a generally straight metaliferrous conductor disposed at a fixed distance from said ion collector which comprises a planar screen.